1. Field of the Invention
This invention relates to improvements in a suspended ceiling grid structure adapted to support ceiling tiles or the like, and more particularly concerns improvements in such structures in connecting means between elements of the grid structure.
The claims of Pat. No. 4,785,603 relate to a seismic-resistant ceiling. The invention claimed in U.S. Pat. No. 4,827,681 and herein are directed to a fire-resistant ceiling.
2. Description of the Prior Art
Seismic-resistant as well as fire-resistant suspended ceiling grid structures for supporting ceiling tiles are known in the art. Typically, such grid structures have a plurality of parallel main beams or runners connected together at right angles by a plurality of cross tees. The main beams and cross tees are made of metal that is generally thin, so that even when the main beams and cross tees are constructed with a double thickness of such metal, the connecting means may fail when subjected to high levels of tension, such as the vibrations caused by seismic disturbances. The metal also has an appreciable positive temperature coefficient of expansion and, consequently, the main beams and cross tees tend to expand from the heat generated by the fires, causing the grid structure to twist and bend and drop the ceiling tiles.
Various efforts have been made in the prior art for modifying suspended ceiling grid connnections means for accommodating the stress of contraction and expansion so as to maintain the proper support of ceiling boards even during a condition of excessive vibration as would be caused by a seismic disturbance or a condition of excessive heat as would be caused by a fire. The prior art proposals to accommodate, and hence avoid, the adverse effects of undue stress in the connecting means of a cross tee grid structure have involved the use of locking tabs to lock a cross tee to a main beam after the cross tee has been inserted through a slot in the main tee. In addition, the cross tees of such a structure have been made to interlock to increase the resistance of such cross tees to forces tending to pull them apart. However, such connecting means in some instances required the use of clips to lock the connecting means together, or a separate end connector made of higher strength steel shaped to the end of the web.
The use of reinforcing clips to lock the connection between cross tees and main beams of a suspended ceiling structure adds undesirably both to inventory and installation problems as well as to cost of materials. The use of these clips in some instances has been necessary, however, because interlocking cross tees and locking tabs of cross tees known in the art have not been strong enough to withstand high compression and tension forces.
Another problem has been how to provide interlocking cross tees with a means for easy disengagement from the interlocked postion for removal of such cross tees without damage.